The present invention relates to an organic thin film electro-luminescence device and an organic thin film electro-luminescence device having pixels arranged in a matrix.
The organic thin film electro-luminescence device has an anode, a cathode and a light emission layer. Holes are supplied from the anode and injected into the light emission layer. Electrons are supplied from the cathode and injected into the light emission layer. As a result, there appears, in the light emission layer, recombinations of the injected electrons and holes. The recombinations of the electrons and holes cause energy-excited states which results in light emissions. The energy-excited states are dependent upon the materials contained in the light emission layer. The organic thin film electro-luminescence device is designed so that an organic light emission layer, showing a strong luminance, is sandwiched between the anode and the cathode. However, depending upon the material constituting the light emission layer, various design modifications of the material constituting the hole transport layer of the organic thin film electroluminescent device are required to obtain optimal luminance efficiency, durability and driving stability. It has been disclosed in the prior art that the organic thin film electroluminescent device, having a hole transport layer and an electron transport layer, is effective in improving luminance efficiency and a driving stability. Further more, doping of guest molecules into the organic emission layer is also effective to improve luminance efficiency and driving stability. In Applied Physics Letters, Vol. 51(12), 21 Sep. 1987, Tang and VanSlyke reported that an organic thin film electroluminescent device exhibited a luminance of over 1000 cd/m.sup.2 and a maximum luminance efficiency of 1.51 m/W at a driving voltage of less than 10 V. Tang and VanSlyke teach that it is possible to develop an organic thin film electroluminescent device that is driven to high brightness at a low voltage, and that the efficiency of the device is dependent upon both the materials and structural features. Namely, it is important that the light emission layer is made of tris(8-hydroxyquinolinolate)aluminum, and further that the hole transport layer placed between the anode and the light emission layer is made of derivatives of triphenyldiamine. The hole transport layer of the derivatives of triphenyldiamine improves the efficiency of the injection of holes into the light emission layer and also determines the probability of the injections of electrons into the light emission layer. As a result, the efficiency in the generation of excitons in the light emission layer is improved. The hole transport layer of the derivatives of triphenyldiamine contributes to confine the generated excitons in the light emission layer.
The improvement of the hole transport layer may not only provide an improvement in the luminance efficiency but may also ensure the driving stability of the above device. It has been proposed that double hole transport layers provide a significant improvement in durability. In this regard, the U.S. Pat. No. 4,720,432 discloses the following. A hole injection layer of a porphyrin system is provided on the anode. A hole transport layer made of derivatives of triphenyldiamine is provided on the hole injection layer. The organic thin film electroluminescience device, having those structures, exhibited a continuous luminance for about 500 hours at an injection current of 5 mA/cm.sup.2 at a voltage of 7.2 V. In the other device without the above structural modifications, the driving voltage is generally around 6V.
In Applied Physics Letters, Vol. 65, p. 807, 1994, Shirota et al. reported as follows. There are provided two hole transport layers. The first hole transport layer, in contact with the anode, is made of -conjugated starburst molecule, 4,4',4'-tris(3-methylphenyl-phenylamino)triphenylamine. The second hole transport layer, in contact with the light emission layer, is made of N,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl!-4,4'-diamine. The combination of those materials may cause the light emission layer of tris(8-hydroxyquinolinolate)aluminum to exhibit a high luminance efficiency. It was confirmed that an initial luminance is 300 cd/m.sup.2 and a half-value period is 300 hours at the constant current driving. The second hole transport layer of N,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl!-4,4'-diamine, provided between the hole transport layer of the -conjugated starburst molecule and the light emission layer, forms a gentle slope barrier, that holes will experience in the injection into the light emission layer. The gentle slop barrier improves the efficiency of the injection of holes into the light emission layer, which, in turn, improves luminance efficiency. In summary, the hole transport layer made of -conjugated starburst molecule, 4,4',4'-tris(3-methylphenyl-pheny-lamino)triphenylamine contributes to driving stability. The second hole transport layer made of N,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl!-4,4'-diamine may contribute rising the efficiency of hole injection, or the efficiency of the luminance.
In Polymer Preprints Japan, Vol. 42, p. 615, 1993, Ito et al. report other electroluminescent device as follows. First and second hole transport layers are in turn provided on an indium-tin-oxide substrate. The first hole transport layer is made of a lyphosphazenpolymer, having as side chains copper phthalocyanine and triphenylamine. The second hole transport layer is made of N,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl!-4,4'-diamine. A light emission layer is provided on the second hole transport layer, wherein the light emission layer is made of tris(8-hydroxyquinolinolate)aluminum doped with quinacridone. After this device was placed in a room for two months, it was continuously driven by a constant current. The device exhibited an initial luminance of 587 cd/m.sup.2 and a half-value period of 166 hours.
In order to develop the organic thin film electroluminescent device, it is important not only to improve the materials for the device, but also to improve the structure of the device. One of the important issues of the device, to be solved by the development, is how to realize a multiple color display with a possible low cost.
One of the organic thin film electroluminescent devices having high fine matrix type display is disclosed in the Japanese laid-open patent application No. 5-253866. The disclosed organic thin film electroluminescent device has a driving circuit, which includes novel thin films transistors serving as switches. This device may suppress the deterioration in the degree of the luminance, caused by a high duty ratio of pulses for driving the device. This means that the organic thin film electroluminescent device, having an increased number of the pixels, may be driven without the deterioration of the degree of luminance.
As described above, the development of the organic thin film electroluminescent device spreads over the improvements of the materials, the structure and the driving performance.
The organic thin film electroluminescent devices described above are designed to be continuously driven by the constant currents. This causes the following problems. First one is a relatively large reduction in intensity of the luminance after a long time passes, thereby the voltage level necessary for the luminance is increased and the luminance efficiency is dropped. As the long time passes, there becomes apparent a variation in intensity of the luminance of the device. One of the cuses of those undesirable phenomenon's is in a crystallization of the material constituting the hole transport layer of the device. The crystallization is caused by heat generated in driving the device. The crystallization is also caused by having a long time pass. The crystallization causes electric field to be applied but not uniformly on the layers between the anode and the cathode. This results in a difficulty in achieving the uniform luminances of individual pixels of the device.
From the above description, it can been seen that, in order to settle the above problems, it is necessary to prevent the crystallization of the material constituting the hole transport layer of the device. It has been proposed by Tsuruoka et al. that a bis-triphenylaminestyryl compound is used as a material of the hole transport layer of the organic thin film electroluminescent device because the bis-triphenylaminestyryl compound possesses a high hole mobility and a high glass-transition temperature. The glass-transition temperature is defined as a temperature, over which the crystallization appears. The high glass-transition temperature may prevent the crystallization, due to the heat generation, of the material constituting the hole transport layer. The high hole mobility is one important factor for causing a high efficiency of injection of holes into the light emission layer. These are reported by Tsuruoka et al. in Japan Chemical Society, 1994. The hole transport layer is so provided as to be in contact with the anode. The emission layer is provided between the hole transport layer and the cathode. This device, when driven at the constant current, exhibits the luminance, characterized by almost the same half-value period as the well known device, having the hole transport layer made of N,N'-diphenyl-N,N'bis(3-methylphenyl)-1,1'-biphenyl!-4,4'-diamine. The above device reported by Turuoka et al. exhibits the luminance at a half intensity, per a unit current-density, of the luminance intensity of the above well known device. The cause of the small intensity of luminance per a unit current-density is in the facts that excitons generated in the light emission layer are quenched but not blocked by the hole transport layer, and that some of the electrons injected in the light emission layer tend to move to the anode side without exhibiting the recombination with holes in the light emission layer.
Accordingly, it would be important for the organic thin film electroluminescent device to ensure a high efficiency of transport of holes in the hole transport layer. It is also important to ensure a high efficiency of injections of holes into the light emission layer. It is further important to secure a sufficiently high degree of confinement, in the light emission layer, of the excitons, generated in the emission layer, and electrons. The sufficiently high degree of confinement of the excitons and electrons ensures a high probability of recombination of the holes and electrons in the light emission layer. It is moreover important to select the optimum material for the hole transport layer, in consideration of the preventions of any crystallization of the material due to the heat generated by the driving of the device.